Automatic test equipment often plays a critical role in the manufacture of semiconductor devices. The equipment provides semiconductor manufacturers with the ability to functionally test each device at the wafer and packaged-device levels. However, with this ability comes cost. Consequently, minimizing test costs is important to semiconductor manufacturers.
FIG. 1 illustrates a conventional semiconductor tester 10, including a computer workstation 12 and a testhead 14. The testhead houses an array of electronic circuit boards, often called “channel cards” 16 that provide the electronic circuitry 18 for individual “channels”. The channels apply signals to and capture signals from one or more devices-under-test (DUTs) 20 mounted on a prober or handler 22. Usually, a manipulator (not shown) carries the testhead above the prober/handler during test. Consequently, the weight of the testhead is an important factor in determining the lifting capability of the manipulator. The cost of a manipulator typically rises with its lifting ability.
One of the problems often encountered when designing the channel cards involves how to cool them. Channel circuitry in modem testheads often generate 5 to 20 watts per chip. Conventionally, liquid-cooled cold plates having formed surfaces configured to match the surface topology of each board were adequate. However, with the tendency to pack more channel circuits into smaller chips on the boards, heat dissipation associated with the chips may overwhelm the cooling capacity for conventional cold plates. Moreover, cold plates tend to be heavy, comprising metal containers mounted to the surface of each board.
One possible alternative to the use of cold plates for cooling high-power circuit boards is to employ immersion cooling techniques. A common conventional technique immerses entire boards into cooling baths of electrically non-conductive coolant. While this scheme works well for its intended purposes, modem channel cards are often fairly large, requiring even larger “baths” to carry out the conventional technique. Moreover, the weight associated with the bath containers do not lend themselves to practical testhead applications.
A proposal described by Suga et al., in U.S. Pat. No. 6,052,284, addresses the bulkiness problem described above somewhat by employing a box-like case around a portion of a board where the channel card electronics are mounted. Each board has electronics mounted on both sides, with respective cases mounted on each side. In this manner, the case may be smaller than the overall board.
While this proposal may reduce the size of the conventional immersion cooled system, the weight involved in having multiple metal cooling cases on each board might be prohibitive for a test head housing between thirty to sixty boards. Excess weight associated with a testhead may require the use of more costly manipulators suited to the heavier loads. In addition, double sided circuitry on a single board limits the number of interconnects that are possible between the integrated circuits.
What is needed and previously unavailable is a lightweight board assembly configured for immersion cooling applications that has an increased interconnect area. The board assembly of the present invention satisfies these needs.